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The installers have been out several times.
1st time they adjusted the gas pipe and said it was too close to the limit switch.
2nd time they fiddled with blower speed.
3rd time they fiddled some more.
4th time they replaced the furnace controller board.
5th time they blamed our 1929 duct work and fiddled some more with blower speed.
6th time they cut a new vent in the main air duct just above the A/C coil. I was told that we'd have to keep that open during the heating season and close it during the cooling season.

As the temperatures slowly dropped this fall, I'd open the vent just a little bit more and that seemed to fend off system malfuctions. And that only worked if we kept the thermostat at a constant temperature setting. If we decided to raise the temperature 2 degrees, you are almost guarnteed system malfuctions before the house heats up.

Now the temperature has hit 12 degrees this AM and with the new vent wide open, closed, and several settings inbetween, the thermostat reports system malfunctions.

The installers say that the 2nd floor works because the air ducts narrow as they go up through the wall cavity from the basement to the second floor and that the return ducts are longer. So that furnace doesn't overheat.

Is any of this true? Is this furnace a lemon? Is the furance undersized and working too hard?